Estimation of Reference Evapotranspiration by Using Different Five Empirical Models for Melkassa Area, Central Rift Valley of Ethiopia

Accurate estimate of reference evapotranspiration is essential for water resource and irrigation project planning and operation that include optimal irrigation scheduling. The study was conducted at melkassa the aim of the study was to estimate reference evapotranspiration of Melkassa area using five different empirical models. Evapotranspiration of the crop and reference evapotranspiration are affected by the climatic parameters like temperature, sunshine duration, humidity, wind speed, solar radiation and so on. Hence, all available climatic data gathered and the estimation of reference evapotranspiration of the area calculated based on five models used. ETo computed by The Blaney-Criddle method, Modified penman method, Radiation method, Throntwaite method and Hargreaves equation. From the result Thornthwaite method gave the maximum rate of Reference evapotranspiration (264.7 mm/month) which is over estimated. The Blaney-Criddlemethod provided the smallest reference evapotranspiration rate (83.7 mm/month). Moreover, the Modified Penman method showed relatively high estimation next to Thornthwaite method and it consists of the energy (radiation) terms and the aerodynamic (wind speed and relative humidity) terms which increased the method to be suggested for the area. The Radiation method showed better evapotranspiration next to Modified Penman method it is the best alternative in the presence of measured wind speed and air humidity data.